AN1769 Freescale Semiconductor / Motorola, AN1769 Datasheet - Page 41

• Manufacturer Part Number: AN1769
• Description: Designing a Minimal PowerPC System
• Manufacturer: Freescale Semiconductor / Motorola

MOTOROLA
8.2 Multiple Scan Chains
JTAG scan chains typically consist of numerous devices to perform in-circuit testing of printed circuit
boards. Since some existing COP controller software may not be able to control the processor if any other
device is present in the scan chain, it is often necessary to provide isolation for the PowerPC JTAG port.
Multiple scan chains is common on complex boards, so this is nothing new; however, for small systems it
may be more desirable to provide an isolation capability that is only created when debugging is desired, and
not while in mass production.
This isolation is shown in Figure 30 and can be done with logic, as in ÒMethod 3Ó, or manually with a
removable jumper or zero-ohm resistor (or even an easily cut PCB trace).
As required by the IEEE 1189.1 (JTAG) standard, even though TMS and TCK will be active when COP
commands are issued, the TDI chain for the rest of the system will ßoat high, causing only IDLE commands
to be issued to all other JTAG devices.
NOTE: Not all emulators assert the present signal. If ÒMethod 3Ó, the logic-controlled method, is used to
separate the scan chain, insure that the chosen emulator will provide the PRESENT signal.
COP
Port
PRESENT
TRST
TDO
TDI
TMS
TCK
Freescale Semiconductor, Inc.
For More Information On This Product,
Minimal PowerPC System Design
Go to: www.freescale.com
Figure 30. COP Isolation
MPC60x
Method 1
0W
resistor
Method 2
Isolation
Method
Jumper
Block
Method 3
LVT08
Logic
Other JTAG
... other
devices
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